LDMOS transistors which are high-voltage power devices have the advantages of a fast switching speed, high input impedance, low power consumption, compatibility a CMOS process, and the like, and is widely used for various power devices such as a display driving IC, a power converter, a motor controller, and a power supply for a vehicle. In the case of power devices, specific on-resistance and breakdown voltage are important factors which significantly affect the performance of such devices. Accordingly, various techniques for increasing the breakdown voltage while maintaining specific on-resistance have been suggested.
Of these suggested techniques, a structure is known in which an internal field ring is formed below the end portion of the gate in the drift region of the LDMOS transistor using a dopant having an opposite type to the drift region.
The breakdown voltage characteristic of the semiconductor device is closely related with the radius of curvature of the source region or the drain region. In particular, the radius of curvature of the source region which is relatively small is one of the important factors for the reduction in the breakdown voltage of the device. As well known in the related art, this is because a phenomenon occurs in which an electric field concentrates on the junction region having a small radius of curvature.
FIG. 1 illustrates a layout diagram showing a power semiconductor device of the related art, for example, an LDMOS transistor.
FIG. 2 illustrates a cross sectional view taken along the line II-II′ of FIG. 1. The same reference numerals of FIGS. 1 and 2 represent the same regions or layers.
As illustrates in FIGS. 1 and 2, the LDMOS transistor of the related art has a protrusion 10′ in a line segment shape or cross-section in a central portion thereof, and includes a source structure 10 which surrounds given regions on the left and right sides and the upper side of the protrusion 10′. The LDMOS transistor includes a drain structure 20 which is formed to surround the protrusion 10′ of the source structure 10. The drain structure 20 is separated from the source structure 10 at a predetermined interval.
The source structure 10 includes a source electrode 11 on the surface of a p-type semiconductor substrate 2, a p-type well region 12 formed in the semiconductor substrate 2 below the source electrode 11, and a high-concentration n+ source region 13 and a high-concentration p+ region 14 which are formed in the p-type well region 12.
The drain structure 20 includes a drain electrode 21 on the surface of the semiconductor substrate 2, and an n-type well region 22 formed in the semiconductor substrate 2 below the drain electrode 21 and used as a drift region. As shown in FIG. 2, the n-type well region 22 is connected to an n-type expanded drain structure 23 which is not covered with the drain electrode 21. The drain structure 20 also includes an n+ drain region 24 formed in the n-type well region 22. The expanded drain structure 23 includes a p-type top region 25.
The gate electrode 30 is formed to be insulated from the underlying channel region by a gate insulating film 40, and the source electrode 11, the drain electrode 21, and the gate electrode 30 are insulated from each other by an insulating interlayer 50.
The LDMOS transistor includes a field oxide film 42 having a Local Oxidation Of Silicon (LOCOS) structure.
As the factors which determine specific on-resistance and breakdown voltage of a semiconductor device having the LDMOS transistor of the related art, there are the length of the drift region and the doping concentration of the drift region. Meaning, a trade-off relationship is exhibited such that, as the length of the drift region increases, specific on-resistance and the breakdown voltage increase, and the concentration of the drift region increases, specific on-resistance and the breakdown voltage decrease. In the structure of the LDMOS transistor of the related art, therefore, it is difficult to increase the breakdown voltage without increasing specific on-resistance.
In the LDMOS transistor of the related art, the breakdown voltage particularly decreases in a tip 10t of the protrusion 10′ in the source structure 10. As a method which prevents this phenomenon, the radius of curvature of the tip 10t can increase, and in this case, it is disadvantageous in that the area of the transistor increases.
As another method, there is a method which decreases the concentration of the drift region in the semiconductor substrate 2 which the field oxide film 42 is formed. In this case, however, an additional ion implantation process is needed, causing an increase in manufacturing cost of the semiconductor device.